We will develop a novel high-sensitivity direct-conversion x-ray detector that could revolutionize digital mammography because it will for the first time allow counting of individual photons leading to "perfect" DQE. Until now all commercial mammography systems have utilized integrating film or detectors. By combining a high-sensitivity detector material, mercuric iodide (H912), with newly available CMOS technology photon- counting readouts, we expect to achieve a significant breakthrough in x-ray detector performance. The new technology will improve the signal to noise ratio by removing the dark current and readout circuitry noise from the signal. These factors limit the sensitivity of integrating mode detectors, but with the ability to detect each individual photon, we can reduce the dose delivered and improve the system throughput without compromising image quality. This is important in digital mammography screening due to the large patient volume. This advance will be made possible by optimizing the characteristics of our direct x-ray converter material, polycrystalline Hg12, a high-Z large-bandgap compound semiconductor, in a new way for use in photon counting. Our efforts have been directed at optimizing the converter film characteristics for use in conventional integrating flat panels. However, because it has outstanding charge collection properties, the material is unique in its potential as a photon counting detector. We propose to develop a process to synthesize spectroscopic-grade high-purity Hg12 and then grow the x-ray converter directly onto specially designed photon-counting readout devices. Newly-developed CMOS readout technology allows us to place a charge-sensitive preamplifier, shaping amplifier, dual discriminators, and digital counter at each pixel in the array. We expect to widely market the device as an OEM component to mammography system manufacturers. In Phase I we will devise the chemical processes to produce the required spectroscopic Hg12 source material, grow the films using thermal vapor transport, test the film performance on glass substrates, and then deposit the film on the CMOS readout devices and test for photon counting and imaging performance. In Phase II we will build a panel detector from modules, modify the Hg12 synthesis and deposition as required, complete a mammography panel detector prototype, and test it at Photon Imaging and the UC Davis Medical Center.